(Grass-Hub isn't a spell.)
Grass-Hub enters the battlefield tapped.
: Target player exiles a card from his or her graveyard.
: Add or to your mana pool.
Put Grass-Hub on top of your library: Add to your mana pool.
: Target player exiles a card from his or her graveyard.: Add 
or 
to your mana pool.
to your mana pool. 
